Re: arm-none-eabi-as: what decides .text section alignment/padding?

[Alexandru Onea <onea.alex@gmail.com>]

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Therefore, is it that the only difference between the two cases shown in my
example boils down to whether the assembler sees an instruction or not?
I understand it as follows:

Case A: .text section with no instruction within, the assembler says "this
monkey asks me to put data in a .text section, whatever, maybe where he
comes from .text means something else"
Case B: .text section with data and at least one instruction, the assembler
says "this monkey now crosses the line, if he wants instructions, I must
make sure that at least other .text sections can be merged into it"

So does the assembler assume that if there are no instructions in a .text
section, maybe it is not actually a .text section in the classical sense?

Best regards,
Alexandru N. Onea


On Mon, 19 Jun 2023 at 16:31, Richard Earnshaw (lists) <
Richard.Earnshaw@arm.com> wrote:

> On 19/06/2023 08:14, Alexandru Onea via Binutils wrote:
> > Hello, community!
> >
> > I am playing with arm-none-eabi-as trying to understand how it aligns
> > sections. I have the following source:
> >
> > ; source.s
> > .text
> > .byte 0xff
> > .byte 0xff
> > .byte 0xff
> >
> > I am inspecting the resulting object file:
> >
> > $ arm-none-eabi-as -mthumb -o source.o source.s
> > $ arm-none-eabi-readelf -S source.o
> > There are 8 section headers, starting at offset 0xec:
> >
> > Section Headers:
> >    [Nr] Name              Type            Addr     Off    Size   ES Flg
> Lk Inf Al
> >    [ 0]                   NULL            00000000 000000 000000 00
> >   0   0  0*  [ 1] .text             PROGBITS        00000000 000034
> > 000003 00  AX  0   0  1
> > *  [ 2] .data             PROGBITS        00000000 000037 000000 00
> > WA  0   0  1
> >    [ 3] .bss              NOBITS          00000000 000037 000000 00  WA
> 0   0  1
> >    [ 4] .ARM.attributes   ARM_ATTRIBUTES  00000000 000037 000014 00
> 0   0  1
> >    [ 5] .symtab           SYMTAB          00000000 00004c 000060 10
> 6   6  4
> >    [ 6] .strtab           STRTAB          00000000 0000ac 000004 00
> 0   0  1
> >    [ 7] .shstrtab         STRTAB          00000000 0000b0 00003c 00
> 0   0  1
> >
> > The .text section is byte-aligned and contains the 3 bytes.
> >
> > Now, I add an instruction to source.s:
> >
> > ; source.s
> > .text
> > .byte 0xff
> > nop
> > .byte 0xff
> > .byte 0xff
> >
> > Looking into the object file, now all of a sudden the .text section is
> > halfword-aligned:
> >
> > There are 8 section headers, starting at offset 0x114:
> >
> > Section Headers:
> >    [Nr] Name              Type            Addr     Off    Size   ES Flg
> Lk Inf Al
> >    [ 0]                   NULL            00000000 000000 000000 00
> >   0   0  0*  [ 1] .text             PROGBITS        00000000 000034
> > 000006 00  AX  0   0  2
> > *  [ 2] .data             PROGBITS        00000000 00003a 000000 00
> > WA  0   0  1
> >    [ 3] .bss              NOBITS          00000000 00003a 000000 00  WA
> 0   0  1
> >    [ 4] .ARM.attributes   ARM_ATTRIBUTES  00000000 00003a 000014 00
> 0   0  1
> >    [ 5] .symtab           SYMTAB          00000000 000050 000080 10
> 6   8  4
> >    [ 6] .strtab           STRTAB          00000000 0000d0 000007 00
> 0   0  1
> >    [ 7] .shstrtab         STRTAB          00000000 0000d7 00003c 00
> 0   0  1
> >
> > What is causing the assembler to decide to pad the section in the second
> > case? I am confused because:
> >
> >     1. if the section is .data then the assembler will not pad it anyway,
> >     which makes sense, but
> >     2. even if the section is .text, the assembler won't pad it unless it
> >     sees an instruction (I can have as many data directives, the section
> won't
> >     be padded without having also an instruction), and finally
> >     3. the nop instruction is definitely not aligned and the assembler
> has
> >     no problem with it, but it still decides to care about section
> alignment.
> >
> > How is the assembler deciding here to align and pad the section? Can I
> > force the assembler to not pad the .text section even if I have an
> > instruction?
>
> You've told the assembler to interpret the contents of the source file
> as containing thumb instructions.  Thumb instructions must be (at least)
> two byte aligned (the fact that your source file deliberately misaligns
> the instruction is a bug in your code).  If the section were not aligned
> then the linker would not be able to correctly merge consecutive
> sections if you had a mix of data-only text sections (like your first
> example) and sections containing code.
>
> Note that if you had used -marm instead of -mthumb the alignment would
> be set to 4 as all 'arm' instructions must be 4-byte aligned.
>
> R.
>
>